It wasn't that because Howland didn't trust Nicholas. He just didn't want Rosie and Nicholas to be out there in the rain. His sister and Nicholas were royal assets, and he couldn't let anything bad happen to them.
"Howland, you worried too much. I just went to the market with Nicholas and we will be back before evening. Besides, we won't be going alone. There will be some guards accompanying us, right?" Rosie said to Nicholas.
"I said the same thing," the man replied with a nod.
"But…"
"Prince Howland, maybe it's too soon, but please entrust your sister to me," Nicholas said, causing Howland to step back.
As long as Aslan hadn't made up his mind and took action, Rosie and Nicholas would be a couple in the future. And as a brother, he had to learn to let go of Rosie's hand and entrust her to the man who would be her future husband.